The present invention relates to an image processing method for processing an electronic image which can be projected onto a projection surface, particularly a screen. Further, the invention relates to an image processing device for performing said image processing method.
Normally, projected images are projected, by means of an image processing device such as, e.g., a projector or video beamer, onto a projection surface such as, e.g., a screen or a smooth white wall. For this purpose, the projection device is arranged frontally towards the projection surface so that the image plane of the projected image on the projection surface and the image plane of the virtual image are arranged parallel to each other. However, it may happen that the projection device has been erroneously or out of necessity placed in a non-frontal orientation in front of the projection surface. In such a case, the image plane of the virtual image is not arranged parallel to the plane of the projection surface so that, since the projection is not performed at a right angle anymore, the projected image will undergo a perspective distortion.
To be able to reverse this perspective distortion of the projected image, it is known, in the field of images existing in electronic form, to electronically distort the electronic image in such a manner that the optical distortion caused by the above outlined conditions will be compensated for. To achieve this effect, it is known from WO 02/101443 to compute an inverse transformation matrix for distorting the electronic image. For this purpose, however, one has to know the exact angle at which the projected beam of a projecting means is incident on a screen or the like surface. On the other hand, this angle is often not known and has to be detected in a complex manner. According to JP 2003 009 038, for instance, the angle is determined by projecting various test images and detecting that test image which results in a distortion-free projected image. To the thus selected test image, one can then assign a concrete angle at which the beam projected from the projection device will be incident on the screen. However, especially if the screen has been rotated e.g. both about a horizontal axis and a vertical axis relative to the projection direction of the projection device, the detecting of the required angle is extremely difficult because this process would necessitate an excessively large number of test images.
According to a known approach for correcting the perspective distortion of the projected image (“keystone correction”), a trapezoidal distortion can be performed. In doing so, the image existing in electronic form, usually being an image of a rectangular shape, is distorted to cause it to assume a trapezoidal shape. The extent of this trapezoidal distortion is set to the effect that the trapezoidal distortion of the electronic image will be reversed by the perspective distortion of the projected image, resulting in a rectangular projected image appearing to the viewer.
For keystone correction by trapezoidal distortion, the rectangular electronic image comprises four image edges which define the shape of the electronic image. Further, the electronic image comprises an image content including a plurality of image elements; these image elements are e.g. partial surfaces of a rendered image. For instance, when effecting a trapezoidal distortion, the right-hand and the left-hand edges of the image are rotated in different rotational senses so that the upper image edge will be compressed and the lower image edge will be stretched, or vice versa. Because of the altered shape of the electronic image, the image content will be newly computed under consideration of all image edges, while the individual image elements of the image content will be stretched or compressed to different degrees, depending on their position within the image.
When performing the above keystone correction corresponding to the trapezoidal distortion, cases where the projection device is arranged obliquely to the projection surface both in the horizontal and the vertical directions will disadvantageously entail the need for two different trapezoidal distortions which overlap each other. Due to this overlap, the operator of the projection device, e.g. a lighting engineer, will already in the first trapezoidal distortion also have to consider the change of the shape of the image caused by the second trapezoidal distortion. This is extremely difficult and prone to failure so that, normally, several trapezoidal distortion steps are required until the perspective distortion of the projected image has been compensated for.
Further, from U.S. Pat. No. 6,361,171 B1, a keystone correction is known wherein the corner points of a rectangular electronic images can be shifted as desired. However, when shifting each individual corner point, it is required to move the corner point into two different coordinate directions. Since the controlling of projection devices is normally carried out via a control panel with several turning switches or slide switches, the user will first have to operate a first switch to move the corner point of the electronic image into a first coordinate direction and then have to change the function of the turning switch so as to move the corner point into a second coordinate direction. Because the movement by which the corner point of the projected image on a projection face arranged obliquely in space is displaced into both coordinate directions will not be performed at right angles, it will be correspondingly difficult to reach the desired end point; thus, several correction steps are required until an individual corner point has reached the desired end point. The same difficulties arise when the operator, using two control switches, moves the corner point both into the first and into the second coordinate direction, entailing the additional disadvantage that the user has to employ both hands so that possible further setting measures cannot be carried out.
It is an object of the invention to provide an image processing method and an image processing device with improved keystone correction.
According to the present invention, the above object is achieved by an image processing method and an image processing device as described in the claims.